In an effort to optimally load data networks, agreements are reached with the users regarding the transmission services to be provided and used, and the pertinent traffic parameters. The data packets are discriminated if the agreement is not complied with, meaning they are discarded in most cases. But since data packets undergo different delays in time during their transportation through the network, different values result with respect to traffic parameters, depending on the location in the network. This situation will be explained in greater detail with the example of data traffic in accordance with the Asynchronous Transfer Mode (ATM).
The ATM comprises different transmission modes, so-called transfer capabilities. They are characterized by their English designations, which are: Constant Bit Rate (CBR), real time Variable Bit Rate (rt-VBR), non real time Variable Bit Rate (nrt-VBR), Unspecified Bit Rate (UBR), ATM Block Transfer (ABT) and Available Bit Rate (ABR). Most of these transmission modes have been standardized by the applicable committees. The network makes these transfer capabilities available to the user for the transportation of his data. After the user has selected an adequate mode for a connection the user and the network agree on the pertinent parameter values. Finally a contract is agreed between the user and the network on the basis of these parameters and the method used to monitor the connection.
Usage Parameter Controls (UPC) are established in certain areas of the network, particularly in the User Network Interface (UNI), or in the Network Network Interface (NNI), where a so-called policing mechanism controls the compliance with the contract. Presently this policing uses a so-called generic Cell Rate Algorithm (GCRA) for all transfer capabilities except for the ABR, with the contract parameters Increment I and Limit L. The Dynamic Generic Cell Rate Algorithm (DGCRA) with the contract parameters Increment I(t) and Limit L is provided for the ABR. But other methods are not excluded according to the ITU-T recommendation. Cells, which is what the data packets in the ATM are called, which do not arrive in accordance with the contract agreements, are discriminated by the policing mechanism, meaning they are especially identified, or in most cases simply discarded, therefore eliminated from the data stream.
As a rule a user complies with the contract by outputting the respective data stream at the terminal. It is also known to use shapers either at the terminal itself or at a network gateway element, for shaping the data stream. As a rule they are based on a policer equipped for this purpose. This is cited for example by EP-A-0711055, which discloses the application of a method to measure characteristic magnitudes in a data stream, and the corresponding device for influencing a data stream. Furthermore in order to influence the data stream, a return message to the source can be provided near the source.
Different treatments in the network influence an ATM traffic stream. The time intervals between the cells of a predetermined connection can therefore be different in different areas of the network. The cause can be the queuing of the data packets originating in the terminal, into a traffic stream in a Customer Premises Network (CPN) located before the ATM network, or statistical multiplexing in the ATM network itself. In ABR traffic this is made worse by the traffic parameters which can continuously change over time. The fluctuations in the arrival time of the data packets of a connection, which are caused by the network, can affect a certain area of the network in a way so that there is always a short-term injury of the contract, even though the user (the source) is complying with the contract. This is particularly fatal in the policing area, where data packets are discarded if their arrival does not comply with the contract.